1. Field of Invention
This invention generally relates to systems for gait retraining. More particularly, the invention relates to a lower extremity exoskeleton designed as a wearable and portable assistive tool for gait neuro-rehabilitation which targets primary gait deviations (e.g. knee hyperextension during stance and stiff-legged gait during swing phase) by reinforcing a desired gait pattern.
Gait can be defined as a person's particular manner of walking. The upright posture assumed during biped walking is unstable in its nature, and walking is oftentimes described as “a continuous forward fall”. Of course, we actually (almost) never fall thanks to the sophisticated coordination of our limbs. Unfortunately, being such a complex process, gait is affected substantially by neurological impairments. According to a 2009 survey, 6.9% of the population in the United States reported an ambulatory disability. The ability to walk and the quality of life are strongly correlated, and therefore restoration of normal gait in the disabled population is of great importance.
2. Description of Related Art
Biomechanics of Walking
It is easy to underestimate the complexity of locomotion, since we walk automatically and with relative ease. Biped gait is a very intricate process that involves the cooperation of several subsystems. From a mechanical point of view, gait is achieved through the coordination of multiple appendages that are actuated by multiarticular muscles that span more than one joint. From a control system point of view, gait involves the dynamic interactions between the central nervous system (CNS) and the peripheral nervous system (PNS).
The human leg has more than seven major degrees of freedom (DoFs), actuated by more than fifteen muscle groups. The human hip is a ball and socket joint with 3-DoFs that allow motion in all three anatomical planes. The knee joint can be simplified as 1-DoF joint that moves along the sagittal plane. Its motions are simply defined as knee flexion/extension. The ankle displays dorsiflexion/plantarflexion in the sagittal plane and inversion/eversion in the coronal plane.
Walking is a quasi-cyclic motion, which can be divided into gait cycles defined as the period between successive heel strikes of the same foot. Generally, the beginning (and the end) of a gait cycle is defined with the ground contact of the same foot. The gait cycle can further be divided into phases and sub-phases. The stance phase is the period where the particular foot is on the ground and supports the body, which constitutes approximately 60% of the gait cycle. The swing phase is where the leg is carried forward, and covers the remaining 40%. In symmetrical gait, both limbs are in contact with the ground for about 10% of the cycle, which is referred to as double (limb) support. FIG. 1 shows the phases and sub-phases of gait.
The gait cycle is useful in studying gait patterns independently from the variations in timing, since it is defined by events rather than time. Variations in joint parameters throughout a complete gait cycle are typically represented by gait trajectories. Even though the gait cycle representation is normalized for time, the gait trajectories are actually influenced by variations in time dependent parameters, such as walking speed and cadence. FIG. 2 shows the mean trajectories of the angle, velocity, and power of the hip, knee, and ankle joints presented as a function of gait cycle (%). Trajectories are shown for six discrete walking velocities ranging from 1 km/hr to 6 km/hr.
Humans achieve energy-efficiency during gait by using gravity as the main driving force. There is a continuous exchange of kinetic and potential energy, as well as energy exchange between different muscle groups. These synergies can easily be compromised by the changes in timing characteristics following a brain or spinal cord injury. Abnormal synergy patterns arise due to lack of control over individual muscle groups. Unintentional co-contraction of antagonistic muscles may also lead to abnormal torque generation at the joints. As a consequence of these changes in the neuromuscular system, various gait abnormalities take place. Reduced comfortable walking speed (CWS) and/or cadence are common in ambulatory patients. In addition, for fear of falling, longer stance phase of the unimpaired leg can also be observed. This results in asymmetrical gait patterns that are inefficient.
The deviations from a healthy gait pattern can be classified into two major groups: primary gait deviations and secondary gait deviations. Primary gait deviations are a direct consequence of the underlying impairment and include stiff-legged gait and drop foot. Stiff legged gait is defined as reduced knee flexion during swing, and is a common primary gait deviation observed in ambulatory patients. The limited toe-clearance poses the risk of tripping over. Stiff-legged gait leads to a gait that is inefficient, unaesthetic, and discomforting. Drop foot is the inability to lift the foot. The ankle flexor muscles are responsible for this motion (or lack thereof). It may cause failure to clear the ground during swing, or result in slapping the ground during heel-strike.
Secondary gait deviations develop as individuals compensate for their primary gait deviations. These compensatory strategies inevitably cause substantial alterations in their gait patterns. For example, limited flexion of the knee and/or the ankle results in insufficient toe-clearance during swing. Secondary gait deviations include hip hiking and hip circumduction. Hip hiking is the excessive elevation of the pelvis during swing. Hip circumduction is the swinging of the leg in an arc (as opposed to a motion confined to the sagittal plane).
Gait Rehabilitation
During the past decade, the field of rehabilitation has witnessed an increasing interest for the clinical use of robotic systems; particularly in the treatment of neurological ailments such as stroke and traumatic brain injury. Stroke survivors typically receive intensive, hands-on physical and occupational therapy to encourage motor recovery. Manual treadmill locomotor training with partial body weight support (BWS) approach has been proven effective in improving gait of poststroke patients. However, manual treadmill training relies on the skill and availability of a physical therapist. Even with the BWS systems, gait training is physically labor intensive. The intensive training required for motor learning is at odds with the availability and cost of a specialized therapist. The scarcity of resources is exacerbated in cases that require a second, or even a third therapist. For instance, for the retraining of a patient who displays hip hiking due to limited knee flexion, one therapist is needed to guide the knee and another one to control the pelvic obliquity. In such cases, it is an additional challenge for the therapists to maintain coordination.
Considering the physical effort involved in such exercises where therapists continually guide the legs and the torso of the patient, robotic neurorehabilitation devices present a great potential as an assistive tool for clinicians by reducing their physical burden. Indeed, some of these systems have already been adopted in clinical practice. Other advantages of robotic systems when compared to manual physical therapy include higher precision and repeatability, and quantitative monitoring of patient's progress via sensors. These factors result in faster and greater level of functional recovery, thus leading to an improvement in patient's level of independence and quality of life.
Maintaining stability during gait is a major concern for most ambulatory patients. In such cases, they are often prescribed stance-control orthotic braces to improve stability. However, conventional orthotic braces such as ankle foot orthoses (AFOs) or knee orthoses (KAFOs) typically address the problem of stability by limiting the patient's range of motion (RoM). Such limitation consequentially instigates abnormal gait patterns. For instance, a KAFO designed to increase stability by limiting knee flexion would result in stiff-legged gait. Because of this primary gait deviation, foot clearance would be compromised during the swing phase. Consequently, the patient would develop compensatory strategies such as hip-hiking and/or circumduction (i.e. secondary gait deviations) to provide ground clearance for the foot. Due to their negative impact on gait patterns, the use of conventional orthotic braces is limited to cases where maintaining stability holds a higher priority than restoring healthy gait patterns. On the other hand, robotic knee orthoses have the potential to overcome the aforementioned limitations by facilitating the knee movement instead of restricting it.
Robotic gait retraining exoskeletons differ from the conventional orthoses at a very fundamental level: robotic gait re-trainers work towards reinforcing a desired gait pattern and reducing the patient's dependence on assistive technologies; whereas traditional orthoses only mask the symptoms. All rehabilitation robots apply forces on the patient's limbs in one way or the other. One such system is a mechanical exoskeleton worn by an operator. Anthropomorphic exoskeletons attempt to mimic the kinematic structure of the human skeleton. As they work in parallel with the user's limbs, mechanical limits can be implemented directly, and the risk of collisions is eliminated. However, the joints should be accurately aligned with that of the user to prevent shear forces. In joints with a single degree of freedom (DoF), such as the knee joint, it is only a matter of adjusting the exoskeleton limb lengths. Unfortunately, proper alignment is not as straightforward in joints that have more than 1-DoF. For example, the human hip comprises a ball and socket joint that is located inside the body. Since it is not physically possible to coincide the exoskeleton's joints with that of the patient's other mechanical solutions are required. For instance, BLEEX utilizes a remote center of rotation (CoR) design. Another design by Herr et al. comprises a cam and roller mechanism that automatically adjusts its length to compensate for the misaligned abduction/adduction joints while still transferring vertical loads.
Several robotic devices for gait retraining of stroke patients have been developed in the last decade. The Lokomat (Hocoma AG, Switzerland) is a exoskeletal bilateral gait rehabilitation robot with a BWS system. The patient's legs are actuated in the sagittal plane via DC motors coupled to ball screws. A spring-based passive foot lifter helps with ankle dorsiflexion during swing. However, the pelvis is only allowed to translate in the frontal plane. Its therapy methods rely on repetition and task-oriented training.
In contrast to Lokomat, LOPES (LOwer Extremity Powered ExoSkeleton) (University of Twente in the Netherlands) is an example of a new breed of rehabilitation robot that is designed to display low mechanical impedance. Low mechanical impedance is achieved via Bowden cable driven joints that utilize series elastic elements. Series elastic elements add mechanical compliance to the system, which renders higher force-feedback gains possible. In addition to the hip and knee joints that Lokomat controls in the sagittal plane, LOPES features additional actuation of the pelvis in the horizontal plane, as well as hip abduction/adduction. These additional DoF are important, since it is known that fixating the pelvis affects natural walking. All motions of the ankle, and vertical translation of the pelvis are allowed, but are not actuated.
The aforementioned devices control multiple degrees of freedom (DoF) of the patient and can only be used in a hospital setting due to their complex design. There exists lower-extremity exoskeletons that are portable, however most are designed to serve as assistive devices for activities of daily living (ADL) and not for gait retraining eLEGS from Berkeley Bionics is such a device that enables paraplegics to stand up and walk with the use of a gesture based human-machine interface, albeit with help of crutches. Another example is the Tibion Bionic Leg. The Tibion Bionic Leg is a fully portable exoskeletal gait retraining powered orthosis with onboard electronics and battery. The Tibion Bionic Leg can aid the patient in sit-to-stand, overground walking, and stair-climbing exercises during which the amount of assistance can be programmed by the therapist via the interface panel on the front. The frame is constructed of composite carbon-fiber material. A pressure sensor at the bottom of the foot provides the control algorithm with the weight-on-foot information. Additional sensors for knee angle measurement, actuator force, motor currents, battery voltage, and internal temperatures are present. The mass of the Tibion Bionic Leg is concentrated around the knee joint, which introduces unusual inertial torques on the hip joint during swing. In addition, there is also no AFO or pelvic brace that help transfer the forces to the ground, and therefore the entire weight of the device is carried by the impaired leg.
The current treadmill-based gait retraining systems like Lokomat, do not facilitate overground locomotion. While existing robotic devices provide a valuable asset for rehabilitation hospitals, their high cost limits the number of training sessions the patients receive during rehabilitation. On the other end of the spectrum, there are examples of portable lower extremity exoskeletons. However, these devices are designed to serve as assistive devices for activities of daily living, and not for gait retraining. Thus there exists a technological gap for a new breed of rehabilitative orthotic devices that maintain the positive attributes of the treadmill devices while downplaying their high cost. In addition, there exists a need for portable gait retraining systems that facilitate overground locomotion rather than just serving as assistive devices for activities of daily living.